1. Field of the Invention
This invention relates to a surface processing method for a steel member accompanied with local melting of the surface of the steel member and to a surface processed steel member having a locally melted and solidified surface layer.
2. Description of Related Art
Along with recent significant improvements in automobile performance, torque converters in power transmission systems of an automobile have also been improved. Torque converters such as shown in FIGS. 1 and 2 provide smoother power transitions than the conventional mechanisms and provide improved gas mileage. A typical torque converter includes a pump impeller 1, a turbine runner 2 forming a torus with the pump impeller 1, a stator 3, a lockup clutch assembly 4, and a damper device 5.
In the torque converter, the engine rotational output through a crankshaft (not shown) is directly coupled to a front cover 6 and the pump impeller 1 fixed to the front cover 6 while the turbine runner 2 is directly coupled through hub 7 to an input shaft (not shown) of an automatic transmission (not shown). When the pump impeller 1 rotates, oil or hydraulic fluid under centrifugal force is circumferentially distributed in the converter torus and is driven by the impeller 1 to rotate with the converter cover or casing around the axis of the torque convertor. The fluid also circulates among the pump impeller 1, the turbine runner 2, and the stator 3. The stator 3 is mounted at its radially inner side on a one-way clutch 31 so as to be rotatable only in a fixed direction and is disposed between the pump impeller 1 and the turbine runner 2. When the pump impeller 1 initially starts rotating and has a large rotational speed difference from the turbine runner 2 such as when the vehicle starts moving, the torque converter operates as a torque converting mechanism to amplify the torque. The rotational speed difference between the turbine runner 2 and the pump impeller 1 decreases as the rotational speed of the turbine runner 2 is increased until the torque converter works simply as a fluid coupling between the turbine runner 2 and the pump impeller 1.
The torque converter includes the lockup clutch assembly 4 to improve gas mileage. That is, when the vehicle reaches a predetermined speed, the lockup clutch piston 41 in the lockup clutch assembly 4 is forced in the axial direction by fluid pressure in response to switching a hydraulic supply by a lockup relay valve or valves (not shown) to engage the piston 41 with the front cover 6 through a friction member 42. The engine rotation is then transmitted directly to the input shaft of the transmission without slippage of the fluid coupling of the torque converter, thereby improving the gas mileage.
The damper device 5 is attached to the torque converter to absorb shocks and vibration caused by abrupt changes of transmission torque occurring when the lockup clutch piston 41 and the front cover 6 are engaged and disengaged. The damper device 5 has input or driving members 50 secured to the lockup clutch piston 41 by rivets 43 formed by hammered heads of protruding portions of the clutch piston plate 41, has a driven plate 51 fixedly mounted on a hub 7 with the turbine runner 2, and has circumferentially disposed springs 52, 53 resiliently interposed between the driving and driven members 50 and 51.
The springs 52 are a first stage and are arranged at eight positions in a circumferential direction of the lockup clutch piston 41; the springs 53 are a second stage and are arranged at four positions in the circumferential direction of the lockup clutch piston 41; the springs 53 are placed inside every other spring 52. The spring 53 has a smaller diameter and is shorter than the spring 52; a helix angle of the spring 52 represents a predetermined set compression value of the spring 53; the spring 53 starts to flex after the transmission torque reaches the value of the bending point torque. When the rotational torque is transmitted from the front cover 6 through the friction member 42 and the damper device 5 to the turbine hub 7, the springs 52, 53 are compressed or flexed to absorb abrupt changes in the transmitted torque during engagement and disengagement of the lockup clutch. The springs 52, 53 can also prevent vibration and noise from occurring due to sudden changes in the output torque of the engine passing to the transmission mechanism (not shown) while the lockup clutch is engaged at higher vehicle speeds.
While the lockup clutch in the above described torque converter is engaged, the springs 52 are compressed by rotation of the lockup clutch piston 41 in a normal driving direction (counterclockwise direction in FIG. 2) relative to the driven plate 51 and are compressed by rotation of the lockup clutch piston in a reverse direction (clockwise direction in FIG. 2) relative to the driven plate 51 when the engine brake is applied or the like to cause the springs 52 to slide repetitively on flat portions 411 of the lockup clutch piston 41. This raises a problem in that the flat portions 411 of the lockup clutch piston 41 wear due to the sliding with the springs 52. Because of centrifugal force from the rotation of the lockup clutch piston 41, the springs 52 are forced radially outward into engagement with a riser portion 412 of the lockup clutch piston 41. During the normal and reverse rotation of the lockup clutch piston 41 relative to the driven plate 51, the riser portion 412 of the lockup clutch piston 42 also slides on the springs 52 repetitively, and is worn as a matter of course.
Generally, one typical prior art approach to prevent such wear caused by sliding actions between different members is to use hard or rigid materials in the members. However hard materials are difficult to mold or fabricate. In particular, hard steel materials cannot be practically employed to form the lockup clutch piston 41, on which plastic working such as pressing molding, riveting or the like is implemented.
Another prior art technique used on steel members is to increase wear resistance by hardening only the surface layer of the steel member. Surface hardening methods such as high frequency hardening, electron beam (EB) hardening, and laser hardening have previously been known. With these methods, the surface of the steel member is heated by high radio frequency energy, laser radiation or a high density electron beam. The heating is stopped when the surface layer of the material reaches a hardening temperature (austenitic temperature) and then the austenite formed at the surface layer is transformed to martensite by rapid cooling by means of self-cooling to form a hard surface layer.
The conventional surface hardening methods require that the surface layer of the material be kept at the hardening temperature by surface-heating for the austenitic transformation time (austenitic transformation completion time) to obtain uniform austenite to implement hardening. As shown in a T-T-A curve diagram for carbon steel in FIG. 3, the hardening temperature must be maintained for a period of time shown by a dotted line (e.g. one second or more) to complete transformation of the steel from ferrite-pearlite to austenite.
Consequently, relatively thin plate materials being hardened by conventional surface hardening techniques will be heated through a substantial portion or all of their thickness because of thermal conductance or heat transfer of the material during the austenitic transformation period; this extensive heating results in deforming the material due to thermal stress, and ineffective hardening of the material due to insufficient self-cooling.